The present invention relates to optics comprising the combination of a single gradient index lens and a single planoconvex lens. More particularly, the invention relates to optics whose numerical aperture (NA) can be increased by combining the two lenses in such a way that the convex surface of the planoconvex lens is opposed to one of the two lens surfaces of the gradient index lens with little or no gap left between the lenses. The optics of the invention is useful as an objective lens in optical heads for reading information recorded on optical disks or writing new information onto optical disks.
Optical disk systems such as a compact disk apparatus and a DVD apparatus use an optical head with which information is read from or written to a recording medium by illuminating the recording surface with laser light that has been condensed with an objective lens to form a beam spot. To increase the density of recording on the medium, the diameter of the beam spot has to be reduced and, to this end, the objective lens desirably has an increased NA on the image side. From the viewpoint of shortening the playback and access times, reduction in size and weight is also important.
Under these circumstances, a plastic aspherical lens having a NA of about 0.45 is conventionally used as an objective lens in compact disk apparatus whereas a glass molded aspherical lens having a NA of about 0.60 is used as an objective lens in DVD apparatus.
In order to further reduce the size of recording apparatus that use optical disks, the objective lens itself has to be made further compact. However, aspherical lenses are manufactured by press working on a die and it is extremely difficult to make tiny lenses having an outside diameter of 1 mm or less (see "Bishokogaku Handobukku (Handbook of Microoptics)", ed. by the Society of optics of Japan, 1995, p. 6).
An example of imaging optics having an outside diameter of 1 mm or less is a rod lens having a gradient refractive index in its radial direction. The gradient refractive index in radial direction of the rod lens is typically represented by the following equation: EQU n(r).sup.2 =n.sub.0.sup.2.multidot.{1-(g.multidot.r).sup.2 +h.sub.4 (g.multidot.r).sup.4 +h.sub.6 (g.multidot.r).sup.6 +h.sub.8 (g.multidot.r).sup.8 + . . . }
where
r: the distance from the optical axis PA1 n(r): the refractive index at distance r from the optical axis PA1 n.sub.0 : the refractive index on the optical axis PA1 r.sub.0 : the radius of the gradient index lens PA1 g: the second order gradient index coefficient PA1 h.sub.4, h.sub.6, h.sub.8, . . . : a higher order gradient index coefficient. PA1 (1) the gradient index lens having a planar or convexospherical lens surface, with its gradient refractive index being expressed as EQU n(r).sup.2 =n.sub.0.sup.2.multidot.{1-(g.multidot.r).sup.2 +h.sub.4 (g.multidot.r).sup.4 +h.sub.6 (g.multidot.r).sup.6 +h.sub.8 (g.multidot.r).sup.8 + . . . } PA1 1.45.ltoreq.n.sub.0.ltoreq.1.80 PA1 0.45.ltoreq.n.sub.0.multidot.g.multidot.r.sub.0.ltoreq.0.90 PA1 r: the distance from the optical axis PA1 n(r): the refractive index at distance r from the optical axis PA1 n.sub.0 : the refractive index on the optical axis PA1 r.sub.0 : the radius of the gradient index lens PA1 g: the second order gradient index coefficient PA1 h.sub.4, h.sub.6, h.sub.8, . . . : a higher order gradient index coefficient; PA1 (2) the planoconvex lens being opposed to the gradient index lens such that its optical axis aligns with the optical axis of the gradient index lens and that it satisfies the following condition: EQU 2.2&lt;n PA1 .theta.: the divergence angle of condensed rays PA1 n: the refractive index of the planoconvex lens. PA1 a: "Hikarikogaku Handobukku (Handbook of Optical Engineering)", published by Asakura Shoten, 1986, p. 306 ff. PA1 b: "Oyobunkogaku Handobukku (Handbook of Applied Spectroscopy)", ed. by H. Yoshinaga, published by Asakura Shoten, 1973, p. 250 ff. PA1 c: "bishokogaku Handobukku (Handbook of nicrooptics)", ed. by the Japan Society of Applied Physics and the Society of Optics of Japan, published by Asakura Shoten, 1995, p. 224.
This gradient index lens is typically manufactured by applying the ion-exchange technology to rod-shaped glass and has the advantage that small-diameter (o.d. .ltoreq.1 mm) lenses can be produced at low cost. In addition, the lens material itself has a positive refractive power, so even a lens of a simple rod shape which is planar on both sides can be used as an objective lens. The refractive power of a gradient index lens is expressed as n.sub.0.multidot.g.multidot.r.sub.0 and the greater its value, the higher the NA of the objective lens that can be produced. However, there is a limit on the difference in refractive index that can be attained by the ion-exchange technology and the value of n.sub.0.multidot.g.multidot.r.sub.0 that can be realized by the ordinary ion-exchange technology is no more than about 0.70.
Comparative Example 1 to be described later in this specification is optics using only a gradient index lens having n.sub.0.multidot.g.multidot.r.sub.0 =0.68 and it gives NA=0.629 on the image side.
However, to realize a higher recording density, the development of objective lens optics having an even greater NA is desired. On the other hand, lenses with high NA are generally prone to experience large aberrations and a certain measure must be taken to reduce them. There also exists the need to reduce the size and weight of the optics in order to make the recording apparatus compact and increase the access speed.
Under these circumstances, the present inventors previously proposed optics useful as an objective lens characterized by the combination of a single gradient index lens and a single planoconvex lens (see Japanese Patent Application No. 203244/1998). The optics was designed on the assumption of using optical glass in the planoconvex lens and it could provide NA of 1.623 on the image side (Comparative Example 2 to be described later in this specification). The proposed objective lens optics has very good optical characteristics. However, the advances in digital recording technology are remarkable and there is a constant need for objective lens optics having even greater values of NA.